The invention relates generally to playback equipment for variable area optical soundtracks and more particularly to an improved apparatus for scanning such sound tracks.
Variable area optical sound tracks on motion picture film have been used in substantially their present form since the earliest days of sound-on-film in the cinema. In their earliest form, a single monophonic optical sound track was used, the width of the clear area being proportional to the recorded modulation amplitude. Later modifications intended to reduce distortion provided for bilateral and dual bilateral tracks adjacent to each other, carrying the same modulation information and being identical in their pattern. A further modification provides for separately modulating the dual bilateral tracks to provide stereophonic reproduction.
Modern commercial film projectors continue to use essentially the same illumination and light sensing arrangements for reading variable area optical sound tracks as those that were used in the earliest sound projection equipment: a light source and narrow mechanical slit to provide a line source illumination, with a single photocell for detection or with dual photocells in the case of stereophonic dual bilateral tracks.
A continuing problem in optical sound track reproduction is the suppression of noise while seeking to provide wide frequency range. Optical sound tracks are particularly susceptible to impulse noise caused by dirt and scratches, which increase with the number of times a print is projected. Other types of noise include grain noise in the white (clear) area of the track and photoelectric cell noise, both of which are essentially proportional to the width of the track being replayed and cause a noise modulation effect when the signal is reproduced.
Various techniques have been introduced to improve the quality of optical sound tracks and indications are that the medium is not inherently as deficient as had been supposed in the 1950's and 1960's, when attempts were made to popularize magnetic sound tracks. A useful discussion of the history and potential of optical sound tracks includes "The Production of Wide-Range, Low-Distortion Optical Sound Tracks Utilizing the Dolby Noise Reduction System," by Ioan Allen in the Journal of the SMPTE, September, 1975, Volume 84, pages 720-729. The paper includes a bibliography in the subject area.
An early attempt at a different approach in replaying the optical sound track is set forth in U.S. Pat. No. 2,347,084 in which the track is repeatedly scanned across its width by a very small scanning spot and detected by a single photo responsive cell to produce an essentially two-level pulse width modulated signal. The signal is limited to suppress noise and is integrated to provide an amplitude varying audio signal. This system is useful in eliminating a substantial amount of noise which is caused by dark spots on the clear area of the track, the black areas being substantially noise-free, except where the negative may have had imperfections, causing white spots. However, in spite of the scanning technique some dark spots on the clear area will still produce noise.
An improved scanning system is described in British patent application No. 37292, filed Aug. 27, 1974 by the present inventor. Said British application corresponds to U.S. application Ser. No. 603,671, filed Aug. 11, 1975 now abandoned which is the parent of continuation-in-part U.S. application Ser. No. 778,870, filed Mar. 18, 1977 and now U.S. Pat. No. 4,223,188. In that system the knowledge that the noise arises primarily in the clear area is taken advantage of by squelching the output signal after initial detection of the black to white transition on the film track during each scan. The reset is effected after each scan. Further refinement of the technique employs a two-way scan so that the white to black transition on a dual bilateral track can be utilized. Signal delays and logic circuits are also employed for this purpose.
In both prior scanning systems described, a problem is in the complexity of the optical scanning mechanisms which would require extensive modification to existing film projectors. Also, long term reliability and relative immunity from frequent adjustments are essential in commercial projection equipment. The prior art scanning systems are susceptible to such problems because of their use of cathode ray tube, mechanical, laser, or other electro-optical scanning techniques.